Frequency synthesizers are used throughout communications systems for tuning the signal frequencies needed for receiving and transmitting. As silicon CMOS technology has been applied at higher frequencies, it has helped the expansion of wireless technology to a wide range of applications.1,2 In particular, these synthesizers have supported applications requiring tuning with fine resolution—from kHz steps to a few MHz—and low phase noise, on the order of -100 dBc/Hz offset 10 kHz from the carrier.

Many of these synthesizers have been developed as integrated-circuit (IC) solutions.3 In terms of circuit architectures, integer-N frequency synthesizers are often challenged in meeting performance requirements such as loop bandwidth, phase noise, and channel spacing due to the fundamental design of the integer-N divider modulus.

In contrast, a fractional-N frequency synthesizer can provide the loop bandwidths needed for many of these emerging wireless applications, with fine channel spacing. In addition, they can achieve low phase noise without excessive reference spurious levels. Since a fractional-N frequency synthesizer uses a higher phase/frequency-detector (PFD) comparison frequency and lower division ratio than an integer-N frequency synthesizer, low-frequency phase noise can be suppressed to a high degree in a fractional-N synthesizer.5

1. This simple block diagram shows the main components of a basic communications transceiver.

Figure 1 represents a typical RF wireless transceiver system, showing the role of the frequency synthesizer in both transmitter and receiver sections.6 Essentially, the frequency synthesizer must cover a required frequency range with adequate output power—as well as acceptable levels of signal integrity and signal purity—with the capability of tuning to meet channel spacing requirements.7 Locking or stabilizing the frequency synthesizer usually works around a specific frequency but, depending upon adjacent components, a synthesizer’s locking loop may favor other frequencies8 (Figs. 2-3).

2. Locking at third harmonic.

3. Locking at adjacent frequency.

For example, harmonic locking can occur when harmonic frequencies have sufficient amplitude levels to engage the synthesizer’s locking loop. This type of locking usually occurs with square waveform modulation where multiples of the desired frequency have sufficient power to cause locking. Long runs of zeros in data bit causes phase detector favors fractional and non-fractional harmonics.9 Side-locking occurs when periodic modulation produces discrete spectral lines with enough energy to cause a synthesizer’s loop to lock to one of these spectral lines. This typically occurs in narrowband frequency synthesizers where the discrete spectral lines are very close, and have high enough amplitude to cause locking.